Use derivatives for distance field antialiasing when available

The current hack to get antialiasing on distance field rendered text in Qt Quick has limitations on perspective transformations, so it does not work well when the text is embedded inside a 3D scene.

On ES 3 (and some ES 2 drivers) we can use derivatives to get the correct antialiasing instead. We need to keep the current code for 2D scenes runnings on older hardware, but when available we should use derivatives instead to increase quality where possible.

(put component Quick: 3D, since this is basically the way of getting text in Qt Quick 3D, so we need it there)